ARM’s performance-boosting big.LITTLE MP could lead to penta-core SoCs

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SoC designer MediaTek has announced the first implementation of ARM’s big.LITTLE MP technology, and plans to pair the capability with an advanced Imagination G6200 Series 6 graphics core as well. The new chip, the MediaTek MT8135, is being billed as a quad-core, even though it’s two Cortex-A15s and two Cortex-A7s running side-by-side. Unlike Motorola’s 8X or Samsung’s Exynos 5 Octa, however, the MediaTek chip really is a quad-core, even if it’s running two very different processors. The key to this functionality is big.LITTLE MP.

To date, ARM’s big.LITTLE power-saving technology hasn’t had the smoothest ramp. Samsung’s Exynos 5 Octa was supposed to be the first SoC to implement the standard, but problems with the chip’s design crippled the Exynos 5410’s implementation and forced Samsung to use Qualcomm hardware in most versions of the Galaxy S4.

When big.LITTLE was unveiled, it was billed as a way for SoC designers to save power by combining low and high-power ARM cores on the same chip. Workloads would then be migrated to the high-power A15 cores if necessary, or kept on the lower-power A7 cores if not. There are three ways to handle this migration: Cluster migration, CPU migration, and Global Task Scheduling. This is what ARM refers to as big.LITTLE MP.

The terms are largely self-explanatory. Cluster migration means that either the A15 or A7 cluster is powered up, but the operating system isn’t really aware of the difference — it sees whichever cluster is running, and nothing else. CPU migration means that each A7 core is paired with an A15 core. This keeps the total number of active cores constant, but allows the OS to use, say, two A7s and two A15s at the same time. Global Task Scheduling-big.LITTLE MP makes the OS aware of all the installed cores and allows it to run tasks on all of them simultaneously.

A system using big.LITTLE MP with a pair of A15s and a pair of A7s, in other words, can run quad-threaded software. It can also use asymmetric core distributions, though MediaTek’s new part doesn’t do this. Still, in the future, the company could theoretically build a penta-core SoC with two Cortex-A15’s and three Cortex-A7 cores.

The reason we haven’t seen big.LITTLE MP to date is because updating the Android scheduler to be aware of all four cores simultaneously has taken time; the functionality isn’t baked into Android 4.3. Presumably, MediaTek plans to work with its partners to ensure they optimize their devices for the Global Task Scheduler (and, by extension, take the greatest advantage of big.LITTLE).

How much big.LITTLE MP boosts performance depends on the application; ARM’s own figures predict gains of 20-60% over just the quad-core alone. The Cortex-A7 cores are still substantially less powerful than the Cortex-A15’s, and just how often the entire quad kicks in will depend on the device’s thermal envelope. MediaTek claims a GeekBench score of 2895 for its CPU, compared to 2530 for the Exynos 5 Octa, 1295 for a quad-core Tegra 3, and 1795 for an iPad 4.

GPU performance is also interesting. According to MediaTek’s claims, the G6200 inside the MT8135 is capable of around 45fps in the off-screen version of the GLBenchmark Egypt test.

That compares to around 40fps for “Competitor A” (that would be the Samsung Exynos tablet, if MediaTek kept the same labels from the CPU test) and around 10fps for Competitor B (the Tegra 3 tablet). Both those figures roughly match what we’ve seen from these devices in other tests. The Apple iPad 4 isn’t shown, probably because it scores around 47fps in the off-screen version of this test.

This is one of the first SoCs to combine support for Global Task Scheduling and a next-generation GPU from Imagination Technologies. If it comes to market on time, it could prove a popular choice for tablet vendors, though whether we’ll see it in any top-flight US designs is an open question. To date, MediaTek’s business design wins have been in Asian markets, but the company wants to push into more powerful designs.

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